1. Field of the Invention
This invention relates to the removal of gases such as sulfur dioxide from the stack exhaust gas.
2. Description of the Prior Art
It is known to use a solution which has been prepared by adding a hydroxide of a metal from either Groups II, III and VIII of the Periodic Table to an aqueous solution of a halide of said metal for the purpose of removing SO.sub.2 from an exhaust gas containing SO.sub.2. More specifically, U.S. Pat. No. 3,386,798 discloses that when an aqueous solution containing 40 to 42.5% calcium-chloride (which will be referred to as CaCl.sub.2) is brought into contact with a stack gas containing SO.sub.2 therein, and thereafter about 0.0013% calcium hydroxide (which will be referred to as Ca(OH).sub.2 hereinafter) is added to the aforesaid aqueous solution, SO.sub.2 is removed from the stack gas. According to the process disclosed therein, CaCl.sub.2 serves to remove SO.sub.2 by reacting with the same, while the Ca(OH).sub.2 is added to the CaCl.sub.2 solution which has contacted the gas containing SO.sub.2 to neutralize or render the CaCl.sub.2 solution weakly alkaline thereby regenerating CaCl.sub.2 therefrom.
However, the patent contains no description as to whether the Ca(OH).sub.2 is contained in the absorbing solution prior to its contact with the stack gas containing SO.sub.2 absorbing solution. In such a conventional process, even if Ca(OH).sub.2 is contained in the absorbing solution prior to its contact with the exhaust gas containing SO.sub.2 there remains the problem that the solubility of Ca(OH).sub.2 in the absorbing solution is limited and as a result, the absorbing capacity for absorbing SO.sub.2, of the absorbing solution per unit quantity will be reduced. Therefore, a need arises to increase the size of the tower in which the stack (exhaust) gas is brought into contact with the absorbing solution, as well as in the amount of the absorbing solution to be used.
In addition, a lime slurry process is known in which an absorbing solution consisting of an aqueous solution of Ca(OH).sub.2 is used. However, this process suffers from the following disadvantages:
1. Due to the lower solubility of Ca(OH).sub.2 in the aqueous absorbing solution, the Ca(OH).sub.2 has to be in a slurry state when it is used, thus causing problems such as clogging during its circulation.
2. Difficulties are encountered when the CaSO.sub.3 which has been precipitated due to the absorption of SO.sub.2 by the absorbing solution is converted to CaSO.sub.4 and this conversion requires that H.sub.2 SO.sub.4 is used to adjust the pH and requires heating and oxidation leading to a costly process.
3. Difficulties are also encountered with the separation of CaSO.sub.3 from the absorbing solution in the slurry state, and it is particularly impossible to separate the CaSO.sub.3 from the absorbing solution in the conventional lime-gypsum process, unless the resultant encompassed or enclosed condition of the Ca(OH).sub.2 by the CaSO.sub.3 is eliminated, this encompassed condition (encapsulating) being created by the formation of CaSO.sub.3 around the Ca(OH).sub.2 due to the use of the Ca(OH).sub.2 in the slurry state as an absorbing solution. In this respect, the solubility of the hydroxides of these metals in water such as for example, that of Ca(OH).sub.2 is extremely low, so low that the hydroxides are present in a slurry state when mixed with water. The low solubilities of the hydroxides not only decrease the absorbing rate of SO.sub.2 gas, but also result in difficulties in the separation of the absorbed SO.sub.2 from the compounds of those metals, such as CaSO.sub.3 or CaSO.sub.4.
For example, in the case of calcium, the absorbing solution is present in the slurry state in which there is formed CaSO.sub.3 or CaSO.sub.4 that tends to encapsulate the Ca(OH).sub.3 (which is the alkaline source). This would not permit separation of the CaSO.sub.3 or CaSO.sub.4 from the Ca(OH).sub.2, i.e., the alkali source is eliminated unless the encapsulated condition of the Ca(OH).sub.2 with CaSO.sub.3 or CaSO.sub.4 is eliminated. This then necessarily causes a certain amount of alkali-loss.
Furthermore, once the absorbing solution reacts with any CO.sub.2 present, the absorbing solution loses absorbing capacity to a degree corresponding to the amount of the absorbing solution consumed in the aforesaid reaction. It is however a general tendency that the absorbing solution will readily absorb CO.sub.2 and thus it is difficult to improve the SO.sub.2 absorbing capability of an absorbing solution. A need therefore exists for a SO.sub.2 absorbing process and solution which is not present in a slurry state and does not suffer from the aforementioned disadvantages.